Microresonator-based optical frequency comb (MOFC) integrates the revolutionary comb precision in a small package for practical applications such as spectroscopy, astronomy, microwave photonics and telecommunication. Most of the research on MOFCs uses the third-order nonlinearity (χ⁽³⁾) of materials to generate a cascaded four-wave mixing effect near the wavelength of the pump light, which are mainly concentrated in the conventional communication band. While in other important bands such as mid-infrared (MIR) band, it is difficult to generate a such OFC due to the lack of a corresponding high-performance laser as the pump light.

Optical parametric oscillators (OPOs) can down-convert the pump laser to longer wavelengths with octave separation via second-order nonlinearity (χ⁽²⁾) of materials, which is widely used for laser wavelength extension including MIR generation. Besides the conventional single-frequency OPOs, the OFC may also be generated in OPOs.

The OFCs in OPOs have been demonstrated, using bulk optics or in monolithic resonators. However, the demonstration for pushing the comb wavelength to the MIR region has not been achieved in a mini-OPO, and high power capacity in a compact system is also important for remote sensing applications.

Fig.1. The schematic and characterization of an optical superlattice box resonator.

Recently, the research group led by Prof. Zhenda Xie from Nanjing University, reported MOFC generation via χ⁽²⁾ optical parametric down conversion process in 2 μm band, which has entered the boundary of MIR. The research results are published in Photonics Research, Volume 10, No. 2, 2022 (Xiaohan Wang, Kunpeng Jia, Mengwen Chen, et al. 2 μm optical frequency comb generation via optical parametric oscillation from a lithium niobate optical superlattice box resonator[J]. Photonics Research, 2022, 10(2): 02000509).

By revisiting the concept of box resonator, we develop a new type of microresonator, optical superlattice box resonator (OSBR), for this realization (Fig.1). The spectral and temporal measurement results agree with our simulation, showing a stable comb generation. The generated 2 μm OFC can be utilized for the sensing of carbon dioxide and water vapor etc., and mine prospect applications.

Such OSBR is fabricated by fine polishing over a lithium niobate (LN) dielectric superlattice, with near-material-limited high quality factor (Q) of 4.0 × 10⁷ at 2096 nm. By pumping with continuous-wave (CW) near-infrared (NIR) laser at 1048 nm, 2 μm comb is generated with comb span exceeding 40 nm. Benefiting from the large mode field of box resonator, the power performance of the generated comb has been greatly improved compared with other microresonstor platforms.

Maximum output power of 340 mW and conversion efficiency up to 12.4 % are achieved. Consistently revival temporal waveforms can be captured, and good comb coherence is presented by narrow beatnote linewidth of less than 20 Hz at a detectable repetition frequency of 1.426 GHz (Fig.2).

Fig.2. The characterization of the generated 2 μm OFC.

In future, the impact of thermal noise on the comb generation can be further reduced by increasing the heat dissipation area and improving the stability of temperature control. More complex structures like chirped structure can be applied to the OSBR for octave spanning OFC generation. Nonlinear dispersion control is possible with proper chirping to manipulate comb dynamics towards soliton comb generation thus higher coherence and stability.

"The new box resonator design is the key for MOFC generation by χ⁽²⁾ optical parametric down conversion process," says Nanjing University professor Zhenda Xie, who led the research effort. "It not only incorporates new physics and comb generation dynamics, but also pushes the comb towards much longer wavelength than pump. This greatly releases the pump laser requirement and simplifies the setup for MIR comb generation, which is important for high resolution spectroscopy, sensing and ranging applications."

Therefore, our χ⁽²⁾ LN OSBR introduces extra freedom for MOFC generation, and similar geometry can be adapted to other platforms, including lithium niobate on insulator (LNOI), for even lower threshold and further integration. We show the advantage of OSBR for MIR comb generation with NIR pump, and open a door for the further studies on these new χ⁽²⁾ MOFC platforms, for their unique dynamics and potential significances in the practical applications.